In Computed Tomography applications, contrast agent concentration maps, such as ‘iodine maps’ which are based on iodine contrast agent or other maps which are based on less common agents such as gadolinium, barium, bismuth or gold, can be very useful in improving the diagnosis accuracy of many diseases and clinical conditions. Two fundamental approaches for generating such iodine maps are known, each method with its pros and cons. One approach utilizes two conventional Computed Tomography scans, with and without contrast agent administration. These scans are also called pre- and post-contrast Computed Tomography scans. By applying volumetric spatial registration followed by image subtraction, the contrast agent map is generated, i.e. the post-contrast image minus the pre-contrast image. The second approach is generating contrast agent map directly from a single spectral Computed Tomography scan. Spectral Computed Tomography, such as dual-energy or photon-counting based, has the ability to quantitatively differentiate contrast agents from biological materials.
There are several advantages of the registration-subtraction method relative to the spectral Computed Tomography technique. First of all, the contrast to noise ratio (CNR) for the same radiation dose is about 3 times higher than in a related dual energy iodine map. This higher CNR can be understood, for example, from the fact that the Hounsfield Unit (HU) difference between the iodine enhancement to the blood or soft tissue HU is about 3 times higher than the difference between the iodine enhancement in the low-energy first-layer image to the high energy second-layer image in dual-layer Computed Tomography (operated at 120 kVp). This comparison includes the consideration that an overall dose equalization between the two techniques brings the original signal to noise of the conventional Computed Tomography image (in the two-scan subtraction protocol) and each of the single-layer images (in the dual-energy single-scan protocol) to about the same level.
Furthermore, in the two-scan protocol, the true non-contrast image is available for clinical diagnostics. This image is usually much better than dual-energy virtual-non contrast image, especially in low-dose Computed Tomography scans.
Besides, in the registration-subtraction technique, the bone and calcium identification and elimination can sometimes be done much more accurately than in single-scan spectral Computed Tomography. This is due to the contrast agent concentration largely varying between the pre- and post-contrast scans as opposed to the bone and calcium remaining the same.
Eventually, dual-energy iodine maps and virtual non-contrast (VNC) images tend to sufferer from significant inaccuracies and artifacts especially in low-dose spectral Computed Tomography scans.
The disadvantage of the two-scan subtraction technique is that a very accurate volumetric spatial registration is required. Although quite good registration algorithms are already available, commercially and in research, the registration results are still not accurate enough in many cases, and miss-registration artifacts exist in the subtraction results.
In addition, performing two scans requires somewhat more complicated and time consuming clinical workflow and patient planning, than just performing a single contrasted scan with spectral Computed Tomography.
A limitation in both techniques is that in order to improve the iodine map appearance, it is common in both methods to apply relatively strong filters to reduce noise and artifacts from the obtained iodine maps, these techniques often degrade significantly the map spatial resolution and the detectability of small features and structures with relatively low iodine concentration.
The purpose of the invention is to combine the best of each existing method to overcome their respective disadvantage.